1. Field of the Invention
The present invention relates to a differential signaling cable used for transmitting high-speed digital signals of several Gbps or more, a transmission cable assembly using the differential signaling cable, and a production method for the differential signaling cable. And specifically, the invention relates to a differential signaling cable in which signal integrity does not deteriorate much, a transmission cable assembly using the differential signaling cable, and a production method for the differential signaling cable.
2. Description of Related Art
In servers, routers, and storage products which handle high-speed digital signals of several Gbps or more, differential signaling is often used for transmission between electronic devices or between boards located in an electronic device. Such electronic devices or boards located in an electronic device are electrically connected by a differential signaling cable.
Transmission of differential signaling uses two signals which have had their phases inverted, and a difference between the two signals is synthesized and outputted on the receiving side. The differential signaling cable is equipped with two signal conductors (also referred to as conducting wire or cable core) to transmit two signals that have had their phases inverted.
Because in a differential signaling cable, currents passing through two signal conductors flow in opposite directions to each other, an advantage is that there is a decreased amount of electromagnetic waves externally emitted. Furthermore, in a differential signaling cable, because noise coming from outside is superimposed equally by two signal conductors, another advantage is that an effect of noise can be eliminated by synthesizing and outputting the difference between two signals on the receiving side. For these reasons, transmission using differential signals is suitable for transmitting high-speed digital signals.
Conventional differential signaling cables include a twisted pair cable in which a signal conductor is covered by an insulator and two of those insulated wires are twisted to form a pair. Since the twisted pair cable is inexpensive, balanced, and easily bent, it is widely used for intermediate-distance signal transmission.
However, because the twisted pair cable does not have a conductor equivalent to a ground, it is easily affected by metals located near the cable and the characteristic impedance is not stable. For these reasons, in the twisted pair cable, there is a problem such that signal waveform is prone to collapse in the high-frequency area of several GHz. Therefore, the twisted pair cable is not often used as the transmission cable when several Gbps or more are to be transmitted.
On the other hand, another type of differential signaling cable is a twin-axial (twinax) cable in which two insulated wires are disposed in parallel without being twisted, and those wires are covered by a shield conductor. In comparison with a twisted pair cable, because in the twin-axial (twinax) cable, a difference in the physical length between two conductors is small and the shield conductor covers the two insulated wires as a whole, the characteristic impedance does not become unstable even when metals are located near the cable, and noise resistance is high. Therefore, the twin-axial cable is used for short-distance (from several meters to several tens of meters) signal transmission at comparatively high-speed (high-rate). Shield conductors for twin-axial cable include conductors using a tape with a conductor (metal foil tape), using a braided wire, attaching a grounding drain wire, and the like.
As an example, JP-A 2002-289047 discloses a twin-axial cable. FIG. 8 is a schematic illustration showing a cross-sectional view of a twin-axial cable as a conventional differential signaling cable.
As shown in FIG. 8, a twin-axial cable 81 is structured such that two insulated wires 84, each made by insulating signal conductors 82 with an insulator 83, are wrapped around or longitudinally supported by a shield conductor 85 which is a metal foil tape made by laminating a polyethylene tape with metal foil such as aluminum or the like, and then the shield conductor 85 is covered by a jacket 86 to protect the inside of the cable. Between the shield conductor 85 and the insulated wires 84, a drain wire 87 is longitudinally disposed so that it comes in contact with the conductive surface (metal foil) of the shield conductor 85, thereby grounding the drain wire 87.
However, in order to transmit high-speed signals of several Gbps or more, it is necessary to reduce skew which is a difference in propagation time of two signals between the two signal conductors. This is because the waveform of digital signals outputted by synthesizing the difference between two signals on the receiving side collapses with increasing the skew. For example, in the transmission of high-speed signals equivalent to 10 Gbps, a skew of only several ps (picoseconds) can deteriorate signal quality. Furthermore, recently, in terms of the necessity for reducing EMI (electromagnetic interference; electromagnetic wave interruption), it is also required to make the differential-to-common-mode conversion quantity low.
Another twin-axial cable is disclosed in JP-A 2001-35270. FIG. 9 is a schematic illustration showing a cross-sectional view of another twin-axial cable as a conventional differential signaling cable. As shown in FIG. 9, a twin-axial cable 91 is structured such that two signal conductors 92 are together covered with an insulator 93, and the insulator 93 is wrapped around or longitudinally supported by a shield conductor 94 which is a metal foil tape, and then the shield conductor 94 is covered by a jacket 95 to protect the inside of the cable. The twin-axial cable 91 makes it possible to suppress a permittivity difference of the insulator 93 and reduce the skew by covering both of the two signal conductors 92 together by an insulator 93.
Still another twin-axial cable is disclosed in JP-A 2007-26909. FIG. 10 is a schematic illustration showing a cross-sectional view of still another twin-axial cable as a conventional differential signaling cable. As shown in FIG. 10, a twin-axial cable 101 is structured such that two insulated wires 104, each made by covering a signal conductor 102 with an insulator 103, are covered by a foaming agent tape 105, and the foaming agent tape 105 is then covered by a shield conductor 106 which is a metal foil tape, then the shield conductor 106 is finally covered by a jacket 107. Between the foaming agent tape 105 and the shield conductor 106, a drain wire 108 is longitudinally disposed so that it comes in contact with the conductive surface (metal foil) of the shield conductor 106. In the twin-axial cable 101, before two insulated wires 104 are covered by a shield conductor 106, they are wrapped with a foaming agent tape 105 functioning as an insulator to keep a relative distance between the signal conductor 102 and the shield conductor 106, thereby enhancing an electromagnetic coupling of both signal conductors 102 and reducing the skew.
Still another twin-axial cable is disclosed in U.S. Pat. No. 5,283,390. FIG. 11 is a schematic illustration showing a cross-sectional view of still another twin-axial cable as a conventional differential signaling cable. As shown in FIG. 11, a twin-axial cable 111 is structured such that two insulated wires 114, each made by covering a signal conductor 112 with an insulator 113 made of a foamed body, are wrapped around or longitudinally supported by a shield conductor 115 which is a metal foil tape, and the shield conductor 115 is then covered by a jacket 116. In the twin-axial cable 111, the insulator 113 is made of a foamed body, and when the two insulated wires 114 are covered by a tape-like shield conductor 115, they are wrapped so tightly that the insulators 113 are slightly deformed in order to make the distance between the two signal conductors 112 small. By doing so, electromagnetic coupling of the two signal conductors 112 is enhanced and the skew is reduced.
As mentioned above, in the twin-axial cable 91 shown in FIG. 9, the skew is reduced by covering the two signal conductors 92 together with the insulator 93. However, by simply covering both of the signal conductors 92 with the insulator 93 as a whole, deviation of the permittivity distribution in the insulator 93 and deviation of the bilaterally symmetric property of the shape of the shield slightly remain. Therefore, effects of sufficient reduction of both the skew and the differential-to-common-mode conversion quantity may not be obtained in some cases when high-speed signals equivalent to 10 Gbps are transmitted.
Furthermore, in the twin-axial cable 101 shown in FIG. 10, since the process of wrapping the foaming agent tape 105 is added, an increase in production costs is inevitable. Moreover, the effects of the skew reduction cannot be obtained unless a relatively thick foaming agent tape 105, such as 0.2 mm thick foaming agent tape 105 is used. Therefore, the bilaterally symmetric property is destroyed depending on the overwrapping condition of the foaming agent tape 105, creating problems in that the skew and the differential-to-common-mode conversion quantity may increase and characteristic impedance may fluctuate. Consequently, it is necessary to precisely control the overwrapping condition of the foaming agent tape 105, however, it is very difficult during the actual process.
In the case of the twin-axial cable 111 shown in FIG. 11, the insulator 113 is deformed by wrapping the two insulated wires 114 with the tape-like shield conductor 115, however, it is difficult to control the distance between the two signal conductors 112, and when the bilaterally symmetric property is destroyed, problems may be created in that the skew and the differential-to-common-mode conversion quantity increase and characteristic impedance fluctuates.
Furthermore, in terms of electrical characteristics, in order to enhance electromagnetic coupling of the two signal conductors, there is a problem such that the desired characteristic impedance (differential impedance) cannot be obtained unless an outer diameter of the cable is made large or the signal conductor is made thin. That is, when the outer diameter of the cable is not changed, the signal conductor has to be made small. Consequently, transmission loss of the cable inevitably increases. On the contrary, when electromagnetic coupling is too strong, in-phase characteristic impedance becomes large. Consequently, characteristic impedance becomes inconsistent with the in-phase input component. As a result, reflection of the in-phase component occurs, which is prone to cause problems such as EMI or the like.
Furthermore, on the mounting surface, in order to enhance electromagnetic coupling of the two signal conductors, it is necessary to make the interval between the two signal conductors relatively small with regard to the outer diameter of the cable. However, when soldering the twin-axial cable onto a board or a connector, the connection pitch becomes small, which tends to make connecting work difficult.
Normally, a drain wire is disposed between the two insulated wires by considering the stability of the bilaterally symmetric property and the position (see, e.g., FIGS. 8 and 10). However, when the connection pitch is small (i.e., the interval between the two signal conductors is small), it is difficult to make connections in their mounting condition, and it is necessary to use a method which peels away a shield conductor to a certain degree and pulls out the drain wire to the edge of the signal conductor and then solders the two signal conductors and the drain wire. Pulling out the drain wire too far makes the grounding unstable, causing electrical characteristics to deteriorate.